High voltage electrical insulator having an insulator body the entire surface of which is covered by a semiconductive glaze

ABSTRACT

A high voltage electrical insulator comprises an insulator body having a pair of upper and lower axially spaced terminals, the entire surface of the body being covered by a semiconductive layer providing a semiconductive path between the terminals, the insulator body providing at least one integral, downwardly depending, bell-shaped portion having an annular rim, said bellshaped portion shrouding a predetermined area of said surface. The geometry of the bell-shaped portion of the insulator body is such that the total protected creepage distance along the shrouded area of the surface is at least as great as the effective air clearance between the insulator terminals, the effective air clearance being the length of the total air gap in the shortest arcing path between the insulator terminals when all but the shrouded areas are wetted. The minimum air clearance between the rim of the bell-shaped portion and the next lower unshrouded surface is at least 3.5 inches.

United States Patent [191 Nigol [4 June 25, 1974 HIGH VOLTAGE ELECTRICAL INSULATOR HAVING AN INSULATOR BODY THE ENTIRE SURFACE OF WHICH IS COVERED BY A SEMICONDUCTIVE GLAZE [76] Inventor: Olaf Nigol, 272 Markland Dr.,

Etobicoke, Ontario, Canada [22] Filed: Dec. 8, 1972 [21] Appl. No.: 313,588

[52] US. Cl. 174/211, 174/140 C, 174/141 C, 174/182, 174/186, 174/196, 174/212 [51] Int. Cl. 1101b 17/50 [58] Field of Search 174/140 R, 140 C, 141 R, 174/141 C, 209, 211, 150, 182, 186, 188,

[56] References Cited UNITED STATES PATENTS 1,661,823 3/1928 Hawley 174/140 C X 1,742,628 l/1930 Barfoed 1,768,948 7/1930 Baum 3,368,026 2/1968 Vince 174/140 C 3,658,583 4/1972 Ogawa et a1. 174/140 C UX FOREIGN PATENTS OR APPLICATIONS 586,065 3/1947 Great Britain 174/140 C 577,253 6/1924 France 174/212 50,462 4/1940 France 174/212 914,141 6/1954 Germany 174/212 310,021 10/1929 Great Britain 174/211 312,205 5/1929 Great Britain 174/212 1,144,430 3/1969 Great Britain 174/150 Primary Examiner-Laramie E. Askin Attorney, Agent, or FirmRid0ut & Maybee 57] ABSTRACT A high voltage electrical insulator comprises an insulator body having a pair of upper and lower axially spaced terminals, the entire surface of the body being covered by a semiconductive layer providing a semiconductive path between the terminals, the insulator body providing at least one integral, downwardly depending, bell-shaped portion having an annular rim, said bell-shaped portion shrouding a predetermined area of said surface.

The geometry of the bell-shaped portion of the insulator body is such that the total protected creepage distance along the shrouded area of the surface is at least as great as the effective air clearance between the insulator terminals, the effective air clearance being the length of the total air gap in the shortest arcing path between the insulator terminals when all but the shrouded areas are wetted. The minimum air clearance between the rim of the bell-shaped portion and the next lower unshrouded surface is at least 3.5 inches.

8 Claims, 9 Drawing Figures aialslesl PATENTEBJUNZS I974 SHEET 2 (IF SHEET 3 0F 4 PATENFEDJURZS m4 PATENTEDJms 1914 SHEET 8 0F 4 HIGH VOLTAGE ELECTRICAL INSULATOR HAVING AN INSULATOR BODY THE ENTIRE SURFACE OF WHICH IS COVERED BY A SEMICONDUCTIVE GLAZE This invention relates to high voltage electrical insulators, and more particularly to insulators for use with high voltage transmission lines and other outdoor systems.

The design of a high voltage electrical insulator for an outdoor system is governed not only by the normal operating requirements of the system, but also by the insulation requirements under transient overvoltage conditions and under unfavourable climatic conditions. Since the insulation requirements and climatic conditions are both highly variable, high voltage insulator design is usually based on experience rather than upon accurate data and analysis of the criteria to be met. This design practice has in most cases provided conservative insulation levels. However, in areas where contamination of the insulator surface from the atmosphere presents a problem it is extremely difficult to provide an insulator which will perform satisfactory even under normal operating conditions of the system. For these reasons it has not been possible hitherto to design insulators so that they would perform reasonably satisfactorily under most conditions to which they would be subjected.

It is an object of the present invention to provide a high voltage insulator which is capable of satisfactory performance under a wider range of operating and climatic conditions than has been possible hitherto with insulators up to twice the overall length.

A high voltage electrical insulator according to the invention comprises an insulator body having a pair of upper and lower axially spaced terminals, the entire surface of the body being covered by a semiconductive layer providing a semiconductive path between the terminals, the insulator body providing at least one integral, downwardly depending, bell-shaped portion having an annular rim, said bell-shaped portion shrouding a predetermined area of said surface.

The semiconductive layer is preferably applied to the surface of the insulator body as a glaze using a glazing composition as described in my copending application Ser. No. 301,340, filed Oct. 27, 1972 and entitled semiconductive Glaze Compositions.

It has been found that the heating effect of the current passing through the semiconductive layer, by raising the temperature of the surface by a few degrees (5C. C.) above ambient and thus preventing the moisture deposition on the surface, increases the flashover strength of a contaminated insulator surface which is exposed to fog and dew by at least five times. This advantage in performance is maintained also under heavy rain conditions, since a predetermined area of the insulator surface is shrouded, by the bellshaped insulator body portion. However, the performance of the insulator under heavy rain conditions is determined by the geometry of the insulator, more particularly by the shape of the interior of the bell-shaped portion and by the spacing of the rim of the bell-shaped portion from the nearest unshrouded surface. These criteria will now be discussed.

The bell-shaped portion of the insulator body is deemed to shroud an area of the insulator structure lying within an inverted right-angled cone which intersects the rim of said portion and is coaxial therewith, this being the area which, in the absence of splashing, remains unwetted by rain falling in any direction at an angle of 45 to the axis of the insulator. The minimum linear distance from the rim to the lower edge of such shrouded area is herein referred to as the minimum air clearance. The minimum air clearance as so defined mustbe greater than the distance through which rain drops can be expected to splash upwardly from a lower unshrouded surface, under extreme conditions. Rain drops splashing upwardly and rain drops held to the rim of the bell-shaped portion would tend to join together under the influence of the electric field and so form a water bridge between the rim and the lower unshrouded surface. In practice it is found that the minimum air clearance, as determined by the maximum splashing distance, must be at least 3.5 inches. In order to provide adequate clearance under extreme icing conditions, the minimum air clearance should preferably be as great as 4.5 to 5 inches, but a value of 3.5 inches is found to be adequate for most climatic conditions encountered in practice. Now the whole of the shrouded area of the insulator surface will not necessarily remain unwetted under heavy rain conditions, because parts of the shrouded area will be exposed to splashing from lower surfaces upon which the rain may impinge. The area which is protected from wetting will generally be less than the shrouded area by an amount determined by the geometry of the insulator and the splashing distance of the rain drops. The protected area of the surface is therefore deemed to be that part of the shrouded area which is spaced from any unshrouded lower surface by a distance greater than the splashing distance. This spacing must be at least 3.5 inches.

Thus the protected area, as distinguished from the shrouded area, is that part of the insulator surface which lies within an inverted right-angled cone intersecting the rim of the bell-shaped portion, and is spaced from the nearest unshrouded lower surface by at least 3.5 inches. The protected creepage distance is herein defined as the minimum linear dimension of the semiconductive layer lying within the protected area, that is to say, the length of the shortest path extending over the semiconductive layer between the boundaries of the protected area. In the case of an insulator having a plurality of bell-shaped portions the total protected creepage distance will be the sum of the protected creepage distances of the separately shrouded areas.

Now the air clearance between the insulator terminals under dry conditions is simply the shortest arcing path between the terminals. This clearance will be effectively reduced by the conductive path lengths provided by the wetted surfaces of the insulator. The term effective air clearance as used herein means the total air gap in the shortest arching path between the insulator terminals under extreme conditions, that is to say, when all but the protected areas of the insulator are wetted.

An insulator in accordance with the invention is characterized by the fact that the total protected creepage distance, as herein defined, is equal to or greater than the effective air clearance between the insulator terminals.

In order that the invention may be readily understood, several embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a view showing in half-sectional elevation two units of an insulator string according to the invention;

FIG. 2 is a diagram of the equivalent circuit of one of the units shown in FIG. 1;

FIG. 3 is a half-sectional elevation, partially broken away, of a second insulator according to the invention.

FIG. 4 is a half-sectional elevation of a third insulator according to the invention;

FIG. 5 is a part-sectional elevation of a fourth insulator according to the invention;

FIG. 6 is a half-sectional elevation, partly broken away, of a fifth insulator according to the invention;

FIG. 7 is a half-sectional elevation, partly broken away, of a sixth insulator according to the invention;

FIG. 8 is a half-sectional elevation of a seventh insulator according to the invention; and

FIG. 9 is a half-sectional elevation of an eighth insulator according to the invention.

DEFINITIONS The terms minimum air clearance, effective air clearance," protected creepage distance, and total protected creepage distance" will be further clarified by reference to the drawings, FIGS. 1, 4, 8 and 9 in particular.

The Minimum Air Clearance is the distance between the tim of a bell-shaped insulator portion and the nearest lower unshrouded surface. This distance is denoted by the distance AB in each of FIGS. 1, 4, 8 and 9.

The Effective Air Clearance is the length of the total air gap in the shortest arcing path between the insulator terminals when all but the protected areas are wetted. In FIG. 1, the insulator terminals are not shown, but the effective air clearance is the sum of the distances AH, assuming that the point H does not lie in a protected area. In FIG. 4, the effective air clearance is the sum of the distances A, H, 42 H In FIGS. 8 and 9 the effective air clearance is simply the distance AH.

The Protected Creepage Distance in FIG. 1 is denoted by the distance KF, the point K (or rather the line K) being distant from the point B by the splashing distance. This is assuming that the distance BG is not less than the splashing distance, but if it is less, then the protected creepage distance will be increased by a small amount below the socket II on the upper surface of the lower insulator. The splashing distance determines the minimum air clearance AB. In FIG. 4 the protected creepage distance for the bell-shaped portion is the distance AF extending along the insulator surface, where the spacing BF is the splashing distance, or minimum air clearance. In each of FIGS. 8 and 9 the protected creepage distance is the distance AF along the inner surface of the bell-shaped portion.

The Total Protected Creepage Distance is the sum of the individual protected creepage distances. For an insulator of the type described with reference to FIG. I, the total protected creepage distance is the sum of the distances KF. For an insulator of the type shown in FIG. 4, the total protected creepage distance is the sum of the creepage distances corresponding to AF. In each of FIGS. 8 and 9, in which there is only one bell-shaped portion, the total protected creepage distance is equal to the protected creepage distance, i.e., AF.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows two insulator elements of an insulator string having an upper terminal and a lower terminal (not shown). Each of the insulator elements comprises a downwardly depending, bell-shaped, porcelain body 10, the body flaring outwardly and providing an annular rim 10a, and a metal cap 11 which is bonded to the insulator body by conductive cement 12. The elements are interconnected by axially extending metal pins 13, the upper end of each pin being located in a well at the top end of the insulating body and bonded thereto by conductive cement, grout and elastic cushion 14. The lower end of each pin is provided with an enlarged portion 15 which engages in a socket 16 provided in the metal cap of the adjacent lower element to be retained thereby. The entire surface of each bell-shaped insulator body, consisting of the inner surface 17a extending from the rim 10a to the conductive cement l4 and the outer surface 17b extending from the rim 10a to the conductive cement 12, is covered by a semiconductive layer which provides an uninterrupted semiconductive path between the metal cap 11 and pin 13. This semiconductive layer is formed by glazing the surface of the insulator body using a semiconductive glaze composition as described in my above identified copending application Ser. No. 301,340.

Each of the bell-shaped insulator bodies 10 shrouds a predetermined area of the surface of the insulator structure so as to shield that area of the surface from driving rain. For practical purposes, each of the shrouded areas may be considered to comprise the total surface lying within an inverted right angled cone which intersects the annular rim 10a of a respective insulator body 10 and which is coaxial therewith. Thus, in the embodiment of FIG. 1, the upper insulator body is considered to shroud all surfaces lying within the inverted cone denoted by the broken lines A, B, C, D, E, including the whole of the inner surface of the upper body and the portion of the outer surface of the lower body lying above a line DB. The minimum air clearance between the rim of each insulator body and the next unshrouded surface, denoted by the distance AB, should be at least 3.5 inches and is preferably 4.5 or 5 inches. This spacing is maintained in order to provide adequate electrical strength under heavy rain and icing conditions. In this embodiment of the invention the protected creepage distance is essentially the total arc length of the generatrices of the surface or surfaces of revolution which define protected area of semiconductive layer lying within the area shrouded by the bellshaped insulator portion 10. The total protected creepage distance, that is the sum of the protected creepage distances defined by the plurality of bell-shpaed portions, should be at least as great as the effective air clearance between the insulator terminals in order to provide maximum electrical strength for a given total length of insulator. With an insulator assembly of this construction in which the electrical strength of the semiconductive layer in the protected areas is 10 kV per inch, the insulator being designed for a 230 kV line with a maximum voltage of 200 kV (crest) to ground and a maximum switching surge voltage of twice the total normal voltage, the protected creepage distance must be about 40 inches. To obtain an adequate impulse strength, which depends upon the effective air clearance between the insulator terminals, six insulator elements should be used, the total length of a six unit string being about one half of the length of a conventional insulator string meeting the same insulation requirements.

The heat produced by electric current in the protected creepage areas must be sufficient to raise the surface temperature by 4 or 5C. above ambient. For most insulator designs this would require a generation of heat of 0.01 to 0.1 watts per square inch. The surface resistivity necessary to generate that amount of heat for different system voltages ranges from about 5 megohms per square to about 200 megohms per square.

An additional effect of the semiconductive layers in the embodiment shown in FIG. I is to increase the selfcapacitance of each insulator body by two or three times the value which it would have without the semiconductive layer. This increase in capacitance can be visualized as the result of extending the physical sizes of the metal cap 11 and metal pin 13 by means of the semiconductive glaze, and in so doing forming a larger capacitor. FIG. 2 shows the equivalent circuit of such a capacitor. From the diagram of this circuit, it is apparent that a higher self-capacitance will result in a lower capacitive reactance and a proportionally higher capacitive current. The components of this capacitive current are shown in FIG. 2 by small arrows. Thus 1,. total capacitive current, amperes where X, capacitive reactance, ohms C total self-capacitance, farads F power frequency, Hz

V voltage applied across the insulator, volts When the self-capacitance of the insulator is increased, say threefold, about two-thirds of the capacitive current flows through the centre position of the semi-conductive layer and consequentially causes some capacitive current heating in addition to the normal conduction heating. The relative importance of the capacitive current heating and the conduction heating depends on the relative values of the total resistance and capacitive reactance of the insulator body. If, for example, the total resistance of the semiconductive layer is much lower than the capacitive reactance at power frequency, the power dissipation and the heating effect are determined largely by the resistance. However, if the resistance is much higher than the capacitive reactance, the power dissipation and the heating effect in the semiconductive layer are dependent on both variables. Again, if two-thirds of the total capacitive current flows through the centre portion of the semiconductive layer, the power dissipation will be where AR resistance of the centre portion, ohms. In this case, the power dissipation and the temperature rise in the insulator are much less dependent on the glaze resistivity. If the glaze resistivity is either low or high, the portion of the total resistance AR appearing in the above expression will adjust itself accordingly so as to maintain a more constant value. The net effect of this stabilization is to widen the tolerance on the resistivity of the semiconductive glaze. For example, suspension insulators of the form described with reference to FIG. 1 having resistance values between 30 and megohms (measured at 20 kV DC) give entirely satisfactory performance when used in strings of six elements each on 230 kV AC lines.

The insulator shown in FIG. 3 consists of a porcelain body 20 having an upper terminal constituted by a glazed surface of high conductivity 21, and a lower terminal consisted by a metal base 22 with a bolt 23. The insulator body provides a plurality of downwardly depending, bell-shaped, portions 20', and 20", each of which shrouds a predetermined area of the surface of the insulator as hereinbefore described. In this case, each shrouded area is considered to be the entire surface of the insulator body lying within each of the inverted, right angled cones which intersect the rims of the bell-shaped portions and are coaxial therewith.

As in the preceding embodiment, the entire surface of the insulator body lying between the upper and lower terminals is covered by a glazed semiconductive layer providing a surface resistivity in the range from 5 megohms per square to 200 megohms per square and an electrical strength of about 10 kV per inch in the protected areas. To provide maximum electrical strength for a given insulator length, the total protected creepage distance along the protected area within the shrouded areas of the semiconductive layer should be at least as great as the effective air clearance between the insulator terminals; and to provide adequate electrical strength inv heavy rain and icing conditions the minimum air clearance between each of the rims of the bell-shaped portions and the next lower unshrouded area of the semiconductive layer should be at least 3.5 inches and preferably 4.5 to 5 inches. The embodiment illustrated in FIG. 4 is essentially similar to the embodiment of FIG. 3, and corresponding parts are denoted by the same reference numerals. In this case, three bellshaped portions 20', 20 and 20", are provided, the entire surface of the insulator body being covered by a semiconductive layer providing a conductive path between the upper and lower terminals of the insulator. The upper insulator 24 consists of a metal cap bonded to the upper end of the insulator body by means of a conductive cement grout and elastic cushion 25. The lower terminal is constituted by a flanged metal base 26, the lower end of the insulator body being located in a well in the base and bonded thereto by a conductive cement grout and cushion 27. The cement grout used in this embodiment, and the other illustrated embodiments of the invention, is rendered conductive by incorporating small amounts of graphite powder and fibres to the grout.

The embodiment illustrated in FIG. 5 is a bushing insulator also of the same basic construction, corresponding parts thereof being denoted by the same reference numerals as in FIGS. 3 and 4. It will be noted that the upper terminal 24 and the lower terminal 26 which form a through connection are of a slightly modified construction, and are provided with metal studs 28 and 29, respectively.

In the embodiment of FIG. 6, the insulator body is constructed as a post consisting of a plurality of porcelain cones or bell-shaped elements, 30, each being flared outwardly at its lower end and terminating in an annular rim 30a. The cones or bell-shaped elements are bonded together by means of a conductive cement grout, indicated at 31, for example, and the upper and lower terminals-of the insulator are constituted by a metal cap 32 and a flanged metal base 33, which are of the same construction as the terminals 24 and 26 shown in FIG. 4. As in the preceding embodiments, each of the elements 30 is considered to shroud an area lying within an inverted right angled cone insecting its rim and coaxial therewith. The total protected creepage distance, represented by the sum of the minimum linear dimensions of the protected areas lying within the shrouded areas of the semiconductive layer, is at. least as great as the effective air clearance between the upper and lower terminals. The minimum air clearance between each of the rims 30a and the next lower unshrouded area should be at least 3.5 inches and preferably 4.5 to inches.

The embodiment shown in FIG. 7 is basically the same as the embodiment of FIG. 6, and corresponding parts thereof are denoted by the same reference numerals. However, this differs from the preceding embodiment in that the shrouding elements 30 alternate with bell-shaped spacer elements 34. The designs illustrated in FIGS. 6 and 7 provide alternative means for obtaining the required air clearances to meet particular insulation requirements.

FIGS. 8 and 9 illustrate insulators in which the insulator body provides just one bell-shaped body portion, 35, each insulator having a lower terminal constituted by a metal post 36 (in FIG. 8) and 37 (in FIG. 9). The upper end of the post 36 or 37 is located in a well of the insulator body which is filled with conductive cement grout 38. The upper terminal of the insulator shown in FIG. 8 is provided by a semiconductive glazed layer of relaively high conductivity, indicated at 39, this layer being substantially the same as that shown in FIG. 3. The upper terminal 40 of the insulator shown in FIG. 9 is constituted by a metal cap having the same construction as the cap 24 shown in FIG. 4. In each case the surface of the semiconductor body is covered by a glazed semiconductive layer providing an uninterrupted semiconductive path between the upper and lower terminals, the layer having a surface resistivity between 5 megohms per square and 200 megohms per square, and having an electrical strength of about kV per inch in the protected areas. As in the preceding embodiments, the minimum air clearance between the rim of the insulator body 35 and the lower terminal 36 or 37 is at least 3.5 inches and preferably 4.5 to 5 inches. this distance is denoted in each of the FIGS. 8 and 9 by the line AB, which line lies in the surface of an inverted right angled cone insecting the rim of the insulator body and coaxial therewith. The protected creepage distance in each case, which is denoted by the distance AF along the inner surface of the insulator body, should be at least as great as'th'e effective air clearance between the upper and lower terminals.

What I claim as my invention is:

l. A high voltage electrical insulator comprising an insulator body having a pair of upper and lower axially spaced terminals, the entire surface of the body being covered by a semiconductive layer providing a semiconductive path between the terminals, the insulator body having at least one integral, downwardly depending, bell-shaped portion having an annular rim, said bell-shaped portion shrouding a predetermined area of said surface, wherein the total protected creepage distance along said predetermined area of the surface is at least as great as the effective air clearance between the terminals, and wherein the minimum air clearance between the rim or each of the rims and the next lower unshrouded surface is at least 3.5 inches.

2. A high voltage electrical insulator according to claim 1, the insulator body providing a plurality of said bell-shaped portions each shrouding a predetermined area of the semiconductive layer surface, said shrouded areas being separated by unshrouded areas, wherein the minimum air clearance between each of the rims and the next lower unshrouded area is at least 3.5

inches.

3. A high voltage electrical insulator according to claim 1, the insulator body providing only one bellshaped portion, wherein the minimum air clearance between the rim and the lower terminal is at least 3.5 inches.

4. A high voltage electrical insulator according to claim 3, wherein the surface resistivity of the semiconductive layer lies in the range from 5 megohms per square to 200 megohms per square.

5. A high voltage electrical insulator according to claim 4, wherein the electrical strength of the semiconductive layer in the protected areas is about 10 kilovolts per inch.

6. A high voltage electrical insulator consisting of a string of downwardly depending, bell-shaped insulator bodies each having an annular rim, the string of insulator bodies extending coaxially between upper and lower terminal conductors and being interconnected by axially extending pins constituting intermediate conductors, the entire surface of each insulator body being covered by a semiconductive layer providing a semiconductive path between an adjacent pair of said conductors, each insulator body shrouding a predetermined area of the surface of said insulator string, wherein the total protected creepage distance along the shrouded areas of the insulator string is at least as great as the effective air clearance between the upper and lower terminals, and wherein the minimum air clearance between the rim of each insulator body and the next lower unshrouded surface is at least 3.5 inches.

7. A high voltage electrical insulator according to claim 6, wherein the surface resistivity of the semiconductive layer lies in the range from 5 megohms per square to 200 megohms per square.

8. A high voltage electrical insulator according to claim 7, wherein the electrical strength of the semiconductive layer in the protected areas is about l0 kilovolts per inch. 

2. A high voltage electrical insulator according to claim 1, the insulator body providing a plurality of said bell-shaped portions each shrouding a predetermined area of the semiconductive layer surface, said shrouded areas being separated by unshrouded areas, wherein the minimum air clearance between each of the rims and the next lower unshrouded area is at least 3.5 inches.
 3. A high voltage electrical insulator according to claim 1, the insulator body providing only one bell-shaped portion, wherein the minimum air clearance between the rim and the lower terminal is at least 3.5 inches.
 4. A high voltage electrical insulator according to claim 3, wherein the surface resistivity of the semiconductive layer lies in the range from 5 megohms per square to 200 megohms per square.
 5. A high voltage electrical insulator according to claim 4, wherein the electrical strength of the semiconductive layer in the protected areas is about 10 kilovolts per inch.
 6. A high voltage electrical insulator consisting of a string of downwardly depending, bell-shaped insulator bodies each having an annular rim, the string of insulator bodies extending coaxially between upper and lower terminal conductors and being interconnected by axially extending pins constituting intermediate conductors, the entire surface of each insulator body being covered by a semiconductive layer providing a semiconductive path between an adjacent pair of said conductors, each insulator body shrouding a predetermined area of the surface of said insulator string, wherein the Total protected creepage distance along the shrouded areas of the insulator string is at least as great as the effective air clearance between the upper and lower terminals, and wherein the minimum air clearance between the rim of each insulator body and the next lower unshrouded surface is at least 3.5 inches.
 7. A high voltage electrical insulator according to claim 6, wherein the surface resistivity of the semiconductive layer lies in the range from 5 megohms per square to 200 megohms per square.
 8. A high voltage electrical insulator according to claim 7, wherein the electrical strength of the semi-conductive layer in the protected areas is about 10 kilovolts per inch. 